All posts by Simon

Most essential if you want to dig a well: gravel pump design

The key tool for sinking a well (unless you want to climb in and dig it out) is a device called a plunscher, or a gravel pump. A punscher is a simple metal pipe, with a rubber flap (valve) at the bottom, and it will fill with sand when pulling it up quickly. This has to be done with enough speed to suck in sand, and you can add water to the well to get this done quickly (plunscher only works when submerged – at least mostly – in water). While this works well for sand and small gravel, the more efficient tool is a gravel pump – essentially a plunscher with a piston inside that will actively suck-in the sand from the bottom of the pipe. The piston will lower again under its own weight, and by repeated pulling it gravel pump will fill with sand and gravel quickly.

This is the setup, you can see the blue rope, make sure to use a good and strong rope, because the forces involved are quite substantial and sand will wear down these ropes, so better to exchange them from time to time in case you want to dig multiples wells.

First modification, a washer mounted in the middle of the rubber flap, it ensures better tightness and valve action. Some plunschers have this feature already from the supplier. Better to use some 4 mm fibre-reinforced NBR rubber.

The mounting screws are difficult to reach, so I mad an extension from a piece of wood…

The weld quality of the plunscher wasn’t all that good, but well, this is not a rocket engine, but a tool fabricated to a certain (rather moderate price tag). This price tag is also the reason why you wouldn’t directly by a gravel pump: a good gravel pump will set you back 150 EUR, whereas a plunscher can be obtained for 45~50 EUR. And, surely, it is more fun to do some modification yourself rather than buying all the expensive tools right away.

This is the modified plunscher, you can see the piston, and the cut-out.

The piston needs to have a valve action as to allow the piston to sink at moderate speed, but obtaining good vacuum when pulling the rope. I also tried to use two ropes: one for the piston, one for gravel pump case, but these ropes get entangled and there is no need for such ropes: the weight of the pump will keep it down, if you just pull with the right force and speed.

The piston needs to maintain some clearance from the wall, otherwise, it will get stuck with small stones, etc.

As a seal, I used leather from an old school bag, very firm and thick leather, and a somewhat smaller rubber disc (fairly hard NBR rubber). This worked well with no significant wear. The leather can be made such that there is almost no gap, for example, 1 mm, to ensure strong suction for fine sand. If you have coarse sand, probably you can also work with a larger gap or worn piston seal, you just need to pull the rope more often. I generally recommend to keep the gap small unless you run into some trouble with specific gravel or some particular sand or stone.

At the bottom, for the last meter, I attached a serrated, rather dangerous-looking teeth ring, to cut into the ground and loosen the sand. It worked marvelously. Generally I can recommend to keep the well flooded as much as possible by adding water all the time, then no sand will flow into the pipe if you hit a somewhat “liquid” layer. I probably kept the water level at least 50 cm above the water table.

The rope needs to be mounted such that the piston cannot be pulled out completely, this needs to be adjusted properly. Sure you could also weld a guide ring or similar for the piston, but it worked out very well with the more “wobbly” piston, the vacuum is strong, and the extraction of sand was more limited by the nature of the lowest sand and clay layer, rather than by the vacuum level.

It seems it wouldn’t hurt for the gravel pump to be a bit heavier, for example, by using a longer and heavy-walled tube, but this will also require more force to lift it up. Also the piston could be a bit heaver to sink more quickly, but well, you will figure out how to operate with the tool after a little while, and a shorter gravel pump is certainly more easily handled. Just make sure to wear proper work boots, because your toe may crack if you drop the gravel pump on it. An the serrated front will bite into your foot as nicely as it bites into hard sand.

As for the diameter, it seem that the 89 mm outer diameter is well suited for a DN115 well pipe. I could imagine that with a larger gravel pump, it may be difficult to withdraw, and will be overly heavy. So unless you gain other experience, any pipe around 90 mm outer diameter will work. I would suggest to use 88.9×5.6 which is a bit heavier, rather than the 88.9×3.6 used for the punscher. But probably all will work if you handle it right.

Down from the well: all kinds of sand

Most important for a well is the nature for the water-bearing layer. Already when using the soil drill I noticed coarse sand when hitting the water layer, and it is so liquid that it can be easily washed down and has almost no turbidity. This is basically a good finding, intermediately coarse sand, with little fines.

The top layer had a few larger stone, but in the water bearing layer, the largest were maybe about 8 mm in diameter.

Interestingly enough, the was a solid but -fortunately- thin layer of sand solidified by white matter in about 3 meter depth (just about 1 cm thick layer). Maybe the river running here over my land dried up some 1000s of year ago?

To color of the sand is somewhat red, but this color doesn’t wash out. Diffing further, from about 6 meters down, gray to black sand appeared. This sand was considerably finer and pretty difficult to remove by plunsching, so I used plenty of water and many strokes of a gravel pump to remove it (I modified the plunscher to a gravel pump by adding a piston – will be described in another post).

Finally, at about 7.5 m down, the sand turned more and more black with some brownish clay fragments and plenty of mica (shiny particles).

Given the 0.3 mm slot width of the filter, about 1-2 mm sand would be quite ideal as a water bearing layer, so I was definitely happy to hit solid clay about 7.6 meters down, and sunk the lowest pipe some 10 cm into this clay layer. Note that the clay layer is really heavy pliable clay, it doesn’t seem to swell or dissolve in water easily. So I even decided not to further close the bottom of the well pipe, it seems soundly stuck and closed by the natural clay.

Some study of the sand revealed that the 7 meter sand has quite some sharp and irregular particles that can clog the filter, so better to keep the filter out of this area as much as possible. Maybe the lowest filter section (2 m in total) is now in the black sand layer for about 0.5 meters.

7 meter sand:

Even more important in such case to not overload the filter, to keep sand from getting into the well by keeping the inlet velocity at well below 0.03 m/s, better 0.02 m/s, which is possible by taking about 2000 L/h through the 2 meters of filter section.

The sand in the main water bearing layer is looking much better, it is coarser, and has more rounded shape.

Red marked are some small gravel, and the red lines show millimeter distances.

Main water bearing sand at about 5 meters, microscopic picture:

For thoroughly removing all sand from the well, I used a sand sucker construction from regular PE pipe (32 mm outer diameter), and a 8 mm pneumatic hose inserted such that it is pointing upwards, and extending about 15 cm inside of the pipe. With ample supply of pressurized air (from a compressor) connected, it will pump up a mixture of water and sand even to 8 meters, no problem. Sure it is a mess of water, sand and dirt, but it is an easy way to get rid of all the fine sand and mica that can’t be effectively removed by the gravel pump finally.

I also supplied plenty of water by a flushing tool, finally, also used this to soften the clay and to flush out a few cm of clay, but introducing a fairly high pressure (6 bars) water jet and pumping out the dirty water at the same time.

I continued to pump out water with the air-operated pump for about 1 hour, finally, I connected an old electric pump to the well for about 3 hour, and during all that time there was basically clean water after the first 30 minutes. And so far it doesn’t show any sand residues after one week of use, so maybe we are safe. Let’s check in one year! Surely I will keep all the tools so I can flush out any sand or residues in coming years should need be.

Drilling and digging: a new well

Since I have moved to my new house, there are extensive gardens that need plenty of watering these days. So far, I have been using a 1984 driven (abyssinian) well, merely a 1-1/4″ steel pipe rammed into the ground. This well has been struggling to provide enough water, less than 500 L per hour. Probably it has reached the end of its life and all attempts to rejuvenate it helped for a while, but I have been looking for a more permanent solution.

That’s the old well – now closed with a cap!

Rather than building again a driven well, which is not quite suitable for the quantities of water that I am looking for, say, 2000-3000 L/h rate, I decided to try a drilled (open) well. Size DN115, which is 125 mm outer diameter, 5 mm wall pipe, specially designed for wells (GWE well pipe, PVC-U, K-series).

The first 80 cm were easily dug with a spade, it is mostly sandy soil with some gravel stones.

Further, I needed to use a soil drill. A neighbor provided it generously, and with some old pipes extensions were made to reach to about 6 meters.

The soil here in the Rhine valley is quite suitable for these kind of drills, in just two hours or so, and with plenty of sweat, the whole reached down to the water table at around 4 meters. Still removing some sand, but you can’t drill into a mixture of water and sand… it will just form a cavity.

There are other kinds of drill, but this is a close-up, a large corkscrew.

Before we proceed, we need to insert the pipe, now assembled to 4.8 meters: 0.8 m sump, 2 meter filter pipe (0.3 mm inlets), 2 meters of plain pipe. It is not all that heavy and I managed to get it in quite easily.

Now, we can already see almost to the center of the earth, at least, 4 m closer to the center…

The pipe needs to be securely mounted so that it can’t move around too much.

Next, we have to deepen the well by a process called plunsching. Bit by bit removing the sand from the inside, and lifting it up with the device, which is basically a steel pipe with a valve at the bottom.

It worked well with coarse sand, but with finer sand, I needed to tighten up the seal and modified it a bit to close tightly. Otherwise the fine sand tends to run out.

Also needed to make a special tool to reach to the screw at the bottom. All a bit inconvenient, but it works.

Also critical is the loading of the pipe, first, I added about 150 kg, later about 240 kg. Easily managed by some old concrete pavers that are about 10 kg each.

The plunscher, I attached it with 3 chain links to the rope, this held it better in place and it could be handled easily.

With up to 350 kg, (the load an my own body weight from time to time), we are well in the save area of the weakest link, the filter pipe.

According to the manufacturer, 2 meters of the filter pipe used should be good enough for nearly 4000 L/h, I may take 2000~2500 L/h, so there is a good safety margin

It is critical to stay below about 0.03 m/s water inlet speed, otherwise there may be effects detrimental to the lifetime of the well.

The cuts in the filter pipe are pretty precise, hard to do this at home.

After about 6.5 meters, things got really difficult, with fine sand, which was also pretty much solidified. I used various tools including water hoses and a mud sucker (a pipe with a PU hose inserted, pointing upwards inside the pipe). The mud sucker uses compressed air (I just supplied the full amount my compressor can generate) and at the top a mixture of sand and water will come up. It is a little mess, but convenient to operate. Also I added plenty of water to the well to keep the level as high as possible, otherwise further sand may be sucked in.

Finally, I reached a layer of clay, and with the help of large quantities of water and air, I managed to dig some 20 cm into it, but it seems really solid and pliable clay.

This scheme shows the well as sunk. it is about half-filled with water, and the suction point is located between the two filters, in an area of no inlets. Ideally, the inlet should be above the filter pipe, but I wanted to allow at least 2 m of water column above the inlet, and with the pump outside the well, the turbulences and local load on the filter pipe will be minimal.

The inlet is just a section of pipe, with many holes drilled into it.

The distribution system and piping as mostly done with 32 mm cold-water PE 100 pipe, connected to legacy 3/4″ piping of my workshop and garden, and some newer pipe (16×2 Pipetec composite pipe).

After only just a few minutes with an old pump to remove dirty water, already the water became nice and clear. Maybe because of the thorough work with the sand pump, there was not much dirt to remove. Also I decided against closing the bottom of the pipe, which now seems to be very solidly embedded in clay anyway.

The water is pouring out plentifully, it is pleasure to the eye and a delight forever!

Everything else could be done easily, just mounting a few pipes and machining a lid from 30 mm PVC plate.

Finally, protected it with some concrete plate and stones while providing easy access for removing water from the pipework in winter, basically, just opening the check valve at the top.

LVDT converter: a Mahr P2004M, some electronics, and sub-micron resolution

Recently, I got a Mahr P2004M linear variable differential transformer (in short: LVDT), which is a device that can measure distance of roughly 2 mm with basically unlimited resolution. As the name says, it is a transformer, and the primary is to be fed with 19.4 kHz (or there abouts) sine, at 5 Vrms, and if the plunger is half-way in, the secondary coils with balance out, and there will be zero voltage. For any displacement from that position, there will be an appreciable voltage at the output. With the right amplifiers and converters, we can use this to measure distances extremely precisely.

To do some test, I mounted the LVDT in a height gauge, because I didn’t know if it was actually working.

The plug was broken, mechanically, but the little board inside was OK. So I replaced the plug, it is rather common 5-pin DIN plug with screw shield, same as is used for precision 100 Ohm Pt100 temperature sensors.

The circuit appears to have some capacitor, resistor, and an overvoltage protection device. I drew the circuit, but nothing special found.

For a basic test, I used a HP 3325B generator and a dual-channel scope.

Clearly seen, the LVDT is working. There is a certain phase shift of the incoming and outgoing signal, which is normal.

The noise is very small, well below 1 mV with some averaging. Note that the signal will probably go through a filter with 1 Hz or slower time constant.

To check the frequency response, I connected the LVDT to a HP 3585A analyzer, and clearly there is a peak sensitivity around 20 kHz. Better to operate close to that frequency (Mahr may specify 19.4 kHz for most of their sensors).

The Mahr datasheet also specifies how the input is supposed to be connected. There is a similar R-C circuit in the plug, at the other end.

Following earlier circuit designs, and also some Application Notes (Analog Devices AN-301 in particular), a circuit has been put together, consisting of a phase-shift oscillator with buffer and stabilized amplitude (TL431 used as a reference).

The key part is the switched rectifier, which is in a fixed (adjustable) phase relationship to the exiting signal. For adjustment, first null the comparator, then adjust the phase shift for precise switching around the zero point and check that this also coincides with the maximum amplitude at reasonable deviation from the zero position (about 1 mm of travel may be good for a 2 mm probe). The adjustment of the phase is fairly non-critical, but will ensure linearity around zero.

For some basic measurement, connected a 16×2 LCD, but finally decided for a 128×64 dot matrix display with white backlight. With that I can use large lettering which is easy to read in the workshop from a distance.

The full schematic, it a bit crude, may need to be re-drawn eventually. There is a power supply, +-15 Volts firstly, for the amplifier circuit, +5 V for the LCD and microcontroller, an ATMEGA128A.

The A/D conversion is done by an ADS1211U (even if the schematic may show ADS1210), a very reliable and highly precise part. A 24-bit sigma-delta converter. These parts don’t come cheap recently, about EUR 30 a piece, but fortunately, I had one in stock.
It has two separate power supplies of 5 V, one for digital, one for analog (with additional filtering): both are derived from the +15 V rail.

The switched rectifier for phase demodulation is done by a DG202 analog switch (all switches paralleled up for low resistance) rather than a FET transistor – simply because this is a way I normally design the lock-in amplifiers and phase detectors.

With everything arranged and tested, I put the circuit in a sturdy aluminum case. The switches are toggle switches that are easy to operate in the workshop. Sure we could attached various touch screens and buttons, but these are not convenient in a workshop with oil and dust.

The little device runs from 230 VAC mains, and doesn’t need much power at all (to most is consumed by the LCD backlight, which is LED based and supplied from the unregulated negative voltage via a resistor current limiter.

Finally, placed the LVDT setup on the granite surface plate.

So far working very well. There is no visible drift, at a 0.1 micron resolution. I have no intention to go below 0.1 micron in my workshop, as this is a metal working facility, no intention to fabricate telescope mirrors or optical parts.

Etalon 77.19000 Height Gauge: a broken pinion

Recently, I found a nice offer on Ebay, an Etalon 77.19000 height gauge, along with a ultra high resolution Mahr LVDT length sensor. About the sensor, we will hear later, but the height gauge, although sold as “working”, didn’t work.

Still in good mechanical shape, and all Swiss Made. Mitutoyo has a very similar model, 192-130, which sells for well over 600 EUR.

The mechanism uses a rack and pinion design, with two pinions, one tensioned by a spiral spring, to avoid backlash. Practically, even without the spring – which was bent – there is no noticeable backlash.

There is also a second rack, and this drivers a fully independent counter mechanism.

After some examination it became clear that one of the pinions had a broken shaft, and this also let to the other pinion spring being damaged. The spring was easily fixed, but the broken shaft of such a tiny and hardened pinion, hard to fixed. I managed to drill a hole with a carbide drill, but when pressing in a new shaft, the whole thing broke apart…. a disaster.

Looking around in my drawers, I would this cheap Chinese dial, 0.01 reading. What if it uses the same rack pitch? Indeed, it does. The dimensions seems to correspond to a module 0.2 gear, and examination under the microscope showed identical tooth count on the pinion. Only the drive gear is a bit different but this could be pushed off.

The gutted dial, well, it is less than 10 USD.

To assembly it, I cut off and ground the lower part of the old pinion to a diameter a little bit less than needed, made a sleeve from stainless steel, and ground a suitable cylindrical length of the spare pinion so that all can be pushed together and fixed in the sleeve to form a new gear assembly of the correct dimensions.

All quickly done with a tool grinder and a lathe. And with the new gear, the Etalon is good as new!

Automated Basement Ventilation: keeping it dry

Basements in older German houses are usually pretty humid and cold, and there are various rules about the proper ventilation. You are supposed to open the window in the early morning, let some dry air in, but during the day, especially in summer, it must be closed. Summer air contains a lot of moisture, because the quantity of water that air can absorb strongly depends on the temperature, the so-called saturation vapor pressure of water. When such moist warm air enters the basement, it will cool down and water will precipitate on the walls. Not good. For me, this is all a bit inconvenient because rather than potatoes I store a lot of electronic parts in the basement, and I want to keep all as dry as possible. So I decided some month ago to set up a little system: (1) a window fan, (2) two humidity/temperature sensors, (3) an ESP32 to control the fan. As sensors, I use the ubiquitous AM2302, because it is easy to read and accurate enough.

For the fan, a KVVR K011301 Model, about 200 m3/h, it also has a feature to close the opening when the fan is off, so that no air can enter the basement when the fan is not running. In principle, you can also use two fans, one to supply air to the basement, and one to extract it, but for me, it works just fine with one fan to extract the air.

A little contraption made to fit the fan to the window. It is all reversible, so if I don’t like to use the fan any more, I can just fit the old window again.

A little control box was quickly made, with an ESP32 module, and a small transformer, a 5 V voltage regulator (be aware that the ESP32 needs pretty high peak current, several hundred mA in WiFi mode! So I needed to add some rather beefy capacitors. Next time I should use a larger transformer…

The key part is the calculation of the absolute moisture level. This is done by regularly (like, every minute) measuring the temperature and relative moisture level inside the basement, and outside, at a protected spot. Then calculate the saturation vapor pressure (which is a formula you can find in textbooks), and multiply with the relative moisture (in percent). This will give you a value that corresponds to the absolute water content (scaling to grams per m3, or similar, but I just use the hPa value, water partial pressure in air). If the water content outside is lower than inside, in absolute terms, I have the controller switch on the fan. There is some hysteresis to avoid all too frequent switching.

A nice box contains all the circuitry, and it can be accessed via a web interface, pretty handy. I also have a server poll the values every few minutes, to prepare some nice diagrams and to check if the system works as designed.

Indeed, it works brilliantly for several months, but then the AM2302 suddenly failed. It would read back the correct temperature, but the moisture value was stuck at 99%. Not good. I tried to clean the sensor, but to no avail. Also, this is the inner sensor, not exposed to the elements or anything. A is outside, B is inside.

So for the time being, I replaced it with a new AM2302, and hope it is just a freak defect and not a general limitation of these sensors — if it is, I will replace the AMS2302 by Bosch BME180 sensor.

This diagram shows the absolute humidity delta, outside minus inside, so if the outside is dryer than inside (for example, outside 10 hPa water, inside 12 hPa, the value will be negative, and the fan will switch on accordingly.

For now, in June, it is all working well, and the basement is indeed much drier than last year, and I don’t need to worry at all about closing and opening windows.

In this region, it seems that at least every few days there is reasonably dry weather for several hours, and the system uses these hours well to ventilate the basement thoroughly. Sure if you have rainy season in your country, this control won’t help, but it all the more moderate climates it seems to be a nice gadget to have.

Workshop Upgrade: Laser cutter and engraver SCULPFUN S9

There are various laser engravers or cutters available in the market, so it is hard to make a choice. Finally I got a Sculpfun S9, which appears to be well-regarded in the community as a cost-effective and capable machine. I was also looking for something that is easily serviceable, not using custom controls or special motors – the Sculpfun S9 is built from all relatively standard components so it can have a very long service life even if I eventually need to fix the electronics or replace the controls altogether.

The machine ships as a kit, but there are good instructions for assembly, step-by-step, even the screws packaged for each step in a separate bag.

The machine is fairly portable, so you can also set it on the surface of large panels to do local engraving or similar.

Some first tests, works very well indeed! Just the smell of burned wood and plastic – it is really not a machine for an apartment, and the laser also seems no toy for kids. It is fairly strong and can be dangerous. This laser has a very sharp (maybe 0.1~0.2 mm beam) that cuts through several mm material in a single cut. No good idea to get your fingers in the way.

You can also cut foil. Maybe better to use a knife cutter (because of the smell and vapors), but for some once-off jobs, it is easy to use and also cuts uneven old foil very well in my test. Better use some magnets to push the foil down on a piece of sheet metal.

The main application that I am looking for is to cut custom seals from plain seal stock. The machine cuts well through 1 mm Elring Abil plate, and similar materials. Even thin rings can be cut, no problem (very hard to cut with a cutter knife or punch).

Next step will be to get the machine properly installed. This means, adding an air nozzle to assist with cutting (made from brass), a machine table, and an enclosure with exhaust fan to get rid of the toxic vapors.

The nozzle is made from a piece of brass (several of these pieces purchased at a scrap yard 25 years ago when I was still a kid).

The nozzle has a side inlet, and is designed for about 20 L/min with 1.5 mm diameter, so we are looking at 150~250 m/s linear velocity at the nozzle outlet.

The gas is fed through a 4 mm PU pneumatic tube.

To measure the air flow, we are using a very cheap Chinese gas flow meter. This has a needle valve, but it is not working well – the needle valve puts a spin on the gas, and the indication is incorrect (the sphere starts oscillating and spinning), so I use a separate needle valve (FESTO GR-QS-8) in the supply pipe.

The flow meters comes with flimsy plastic connectors, and these have 5/16″ UNF (24 TPI) tread… not a very common part in Germany to get a transition from 8 mm pneumatic tubing to 5/16″ UNF…

Fortunately, found a suitable thread cutter in my tool selection, so an adapter was made quite easily (from 5/15″ UNF to 1/4″ NPT, then us a 1/4″ NPT to 8 mm push-in tube connector).

The next step has been to make a suitable table, sure you can put the laser machine on some ordinary table, but it has quite some speed and movement and even relatively stable tables will be moving and there is some impairment of precision. So I wanted to give this machine a stable basis that doesn’t shape. It is all welded construction from about 1.5″ square steel pipe. The top is 18 mm waterproof plywood. There will be a piece of zinc-plated sheet metal on top, also to use magnets, and a open cutter support plate on top in case of heavy cuts.

To fabricate such a table, after the welding is done, grind off the scale (this is just plain steel), clean with acetone, and then roll-on some primer paint.

Finally, painted in a blue-gray color, and with the top plate mounted.

Next step, fabrication of an enclosure with a movable cover. All made from 15×15 mm (about 3/4″) square tube, all TIG welded…. looks easier as it is with all the parts and angles.

There will be two large windows, 40×60 cm, to observe the process. I selected GS-1C33-GT Plexiglas (acrylic glas) which is nicely transparent for visible light, but blue light of the laser can’t pass at all.

This is also confirmed by the spectrum, the laser is emitting at about 452 nm.

After some hours of work, the enclosure is ready for painting. It opens nicely and smoothly, also because of a gas spring (200 N, 535 mm total length, 220 mm travel).

It is one of the few occasions that I have use gas loaded springs in my design but it is working well. Just the design calculation is complicated and probably it is always a bit of experimentation to find the right gas spring size and force. But this time, successful at the first attempt. My recommendation, to rather use a slightly stronger spring (say, 200 N if 150 N is calculated) to allow for aging of the spring or other design uncertainties.

Workshop Upgrade: 3D Printer

Finally, I had to opportunity to acquire a long desired tool, a 3D printer. In recent years these have become fairly affordable, and also easy to use. So it is on longer needed to spend days with optimizing various settings. I am planning to print mostly in durable parts, so I am targeting PETG and ABS rather than PLA plastic materials. Mostly planning to use it for spare parts, or mold patters for aluminum casting.

The machine, it is an Artillery Sidewinder X2, a great product, all nicely arranged in a box.

It didn’t take long to set it up, maybe one hour, and then you can use the well known software packages to run the machine. I am using Freecad for modeling and Cura for processing.

All worked fine already for the first part… great!

A cylinder, and a space shuttle. All fairly robust.

So far, I can only praise the machine, it is working fast and precisely. Just printing low cost PETG material. Key point is to keep the bed rather hot to keep the parts sticking, and then just remove them after cool-down with no effort.

Cheap personal scales: turned into parcel scales with USB interface

With many parcels being shipped, and for some other projects including measuring the contents and consumption of LPG gas cyliners, other gas cylinders, chemical tanks etc, I always wanted a slim and stable balance, at low cost. Sure we can fabricate one from steel plates and load cells, at considerable cost. But why not try with a personal scales, and convert it to some usable tool. This scales from local LIDL supermarket comes for EUR 8.99 in the shop, 10 dollars.

It is very slim and stable, basically a piece of hardened glass, with 4 load cells. There are 2 thin-walled stainless tubes to carry the wires to the main processor.

That’s the type, just for reference:

The load cells are the typical 3-wire elements (two load elements inside, red is the center).

The main disadvantage of these balances is (in addition to the absence of an interface) the absence of a continuous reading, in contrast to a good old analog balance it only shows one weight once, when you step on it. For various uses, I rather need a balance that continuously shows the weight, and can transfer it to a host for data analysis.

The load cells rest on certain plastic parts that are glued to the glass plate.

We need to cut a little modification on the milling machine, to make space for a small AVR controller board, and the USB (micro-USB) plug. There is no need to batteries any more, it will all be powered by USB.

For the load cell interface, the 4 load cells need to be wired up in a bridge (not important in which order of the load cells, but always white-while black-black, and alternating red wired for drive and signal inputs.

So we use a very common HX711 driver, it seems to work well with these load cells. I still had a board with higher data output rate (can be changed by floating a pin on the HX711), but you may select the data rate as you like. The HX711 is continuously active, and sending data to the host though a USB serial chip, CH340.

The internals, a arduino nano fake board (not running arduino code, but just some plain avr-gcc code), and the HX711 all wired up, we just need some hot glue to keep it together.

All ready to be put back together. Watch the wires, these are very thin.

The USB port, it all looks as if it had never been without.

The receiver side, the balance digital converted value is directly transmitted to the host, so all the conversion to kg and zero correction is done by the host, including smoothing.

It is just a simple software, I programmed it using wxWidgets, but you can use any software that can handle USB/RS-232 communication and read serial ports, including Python, etc.

One bug with Windows – because of the continuous data stream over serial, Windows 7 seems to believe the balance is a serial mouse, and starts randomly moving the mouse pointer. With the little code below, this can be disabled and avoided. Probably need to add some waiting time before transmission starts in the AVR code.

If you need any of the code or further instructions, just let me know!

For calibration, a good hint, you can use milk packages (UHT milk), one pack is about 1060 g with very little variance, and it can easily be stacked up.
The Zero-Point seems to be very stable, I tested it after 90 kg load changes and so on, and it is not moving by 10 g.

CNC Lathe Signal Conditioning: fighting the noise

For the control of my CNC lathe, I need to read the spindle encoder (index and rotation signal), and two glass scales. These encoders all provide TTL level signals, and so far it worked well, but there are cases were electric noise of the switching motor or other equipment in the workshop. So finally I decided to put a bit more effort and upgrade the interface.

So far, it is just a cable that connects the three sources (spindle-index, x, z) to a parallel port input (a PCI card).

For signal conditioning, the input signals are filtered with a 4n7 capacitor+330 R, followed by a 74LS14 inverter schmitt trigger.

Everything on a little PCB, and put in dust-proof case so that it will work in the dusty workshop.

Finally, put it to a test and it worked “out-of-the-box”.

A simple schematic: